1. Technical Field
The present invention relates generally to plasma-producing devices, in particular to electron cyclotron resonance plasma-producing devices. More particularly, the present invention relates to electron cyclotron resonance plasma-producing devices employing a combination of waveguide structures and permanent magnet assemblies.
2. Description of the Related Art
Plasma-producing devices are commonly employed in microelectronic device fabrication and similar industries requiring formation of extremely small geometries. Plasma-producing devices may be utilized in plasma-assisted processing to etch geometries into a substrate or to deposit a layer or layers of material on the substrate.
One class of such plasma-producing devices employs a magnetic field in conjunction with microwave energy. In these devices, plasma is produced from a working gas as a result of the inter-action of a magnetic field with an electric field. A microwave waveguide may be employed to inject microwaves, which have an associated electric field, into an evacuable chamber containing the working gas. The microwaves propagate into the chamber in a direction substantially perpendicular to the surface of the workpiece. The electric field associated with the microwaves is perpendicular to the direction of propagation, radially outward from a line following the direction of propagation of the microwaves. Plasma ions from the working gas are accelerated by the electric field along such radial lines.
A magnetic field is provided close to the point of injection in a direction generally aligned with the direction of microwave propagation, causing plasma electrons within the working gas to rotate around the direction of microwave propagation at right angles with the magnetic field. At the plane of resonance, the point at which the electric field associated with the microwave energy and the rotation of plasma electrons are in phase, the microwave electric field constantly accelerates the rotating plasma electrons. The energy of this acceleration dissociates molecules of the working gas into atoms and removes electrons from the atoms, creating ions and additional electrons. The ions then diffuse and impinge upon the exposed surface of the workpiece.
The requisite magnetic field may be provided by a single permanent magnet situated above the outlet of the microwave waveguide into the chamber. An adjusting element may be provided to vary the spatial relationship between the magnet and the waveguide opening, thus altering the location of the plane of resonance or "resonance zone" within the chamber.
Plasma uniformity across the surface of the workpiece is generally necessary to achieve etched geometries or deposited layers having relatively uniform dimensions from the center to the periphery of the workpiece surface. Prior art attempts to obtain plasma uniformity focusing on achieving a uniform magnetic field require very large and bulky magnets. Another drawback of the use of permanent magnets in plasma-producing devices relates to the necessity of positioning the microwave waveguide between the permanent magnet and the workpiece. This constrains placement of the permanent magnet with respect to the chamber, and as the magnet face is moved further from the chamber, larger, more expensive magnets are required to produce the requisite magnetic field.
In these plasma-producing devices, also referred to as electron cyclotron resonance plasma systems, the electron cyclotron resonance absorption occurs in a region of magnetic field strength where the gyromotion of the electrons is resonant with the excitation frequency. A fairly typical drive frequency is 2.45 GHz, which is resonant at a field of 875 G.
Microwave energy transmits from a source (at atmosphere) to the plasma (at vacuum) through waveguide structures. A microwave window, transparent to microwave energy, is required to separate the vacuum from atmosphere. This vacuum window is typically made of quartz. During processing, this window is exposed to the plasma process and may be either etched or subject to deposition, depending on the type of processing. Typical waveguide structures used include the standard rectangular or circular cross section waveguides and coaxial structures.
Efficient coupling of the microwave energy into the plasma requires that the load be matched to the source. The structure that couples the waveguide to the plasma chamber and plasma is termed a "launcher" or "coupler". The coupling of the waveguide to the plasma chamber should also provide for a large area of uniform or symmetrical plasma generation. The field distribution coupling from the waveguide to the plasma should control the plasma uniformity to a large extent. This means uniform over an area as large as the workpiece.
In practice, the plasma production region connects to or feeds a larger process chamber in which the magnetic field directions or diffusion (perhaps within a magnetic bucket structure) acts to enhance uniformity. In this case, the plasma production region may not need to be of a large area, however uniformity remains a desirable trait. Usually uniformity over a disk region is desired. However, uniformity over an annular region or a region of field with mirror symmetry may be useful. In the case of uniformity over an annular region that produced by a circularly symmetric field distribution, the remaining structures may act to homogenize the annular plasma to a uniform disk at the workpiece. A field distribution with a mirror symmetry may still produce a time averaged circularly symmetric plasma, provided the mirror symmetry plane rotates with time.
The launcher is a matching device to efficiently transmit the microwave energy from the source to the vacuum vessel. Once transmitted into the vacuum vessel, the microwave energy will be absorbed by the plasma. For a given frequency (or plasma density) and magnetic field, only certain polarizations of electromagnetic waves will propagate in a plasma. Others will be absorbed or reflected. The electromagnetic waves are required to propagate to the resonance zone. A wave launched from low density and at a low magnetic field can not propagate to the high density. It is essential that the microwaves enter the chamber at a high magnetic field.
Generally, different modes will be absorbed and/or reflected by the plasma differently. Launching of modes that are absorbed by plasma enhance the tuning as well. Based on the above, a launcher should 1) send the waves into the chamber at a field greater than that required for resonance; 2) obtain a propagation parallel to the magnetic field in regions of high density; and 3) launches the appropriate field distribution mode. Therefore, it would be advantageous to have an improved electron cyclotron resonance plasma-producing device with an improved launcher for directing microwave energy into plasma in a chamber at a high magnetic field.